The applications of fiber-optic sensors are diverse. Mention may be made of the publication by B. Culshaw entitled “Optical fiber sensor technologies: opportunities and—perhaps—pitfalls”, J. Light. Tech. Vol. 22, No. 1, 39 (2004). The most frequent applications relate to stress, temperature and pressure detection, but there are also applications in the field of detecting current/voltage, displacement, torsion, acceleration, gases, etc. The techniques employed are very varied, the ones most actively studied relating to:                fiber gyros (see, on this subject, V. Vali and R. W. Shorthill, “Fiber ring interferometer”, Appl. Opt. Vol. 15, No. 5, 1099 (1976));        other interferometric methods (see P. Nash, “Review of interferometric optical fiber hydrophone technology”, IEE Proc. Radar Sonar Navig. Vol. 143, No. 3 (1996)); and        backscattering techniques such as Raman, Brillouin or Rayleigh scattering. The reader may refer, in particular, to L. Thévenaz et al., “Monitoring of large structures using distributed Brillouin fiber sensing”, Proceedings of the 13th International Conference on optical fiber sensors (OFS-13), Korea, SPIE Vol. 3746, 345 (1999).        
Almost half of the fiber sensors currently studied employ Bragg gratings (S. W. James et al., “Simultaneous independent temperature and strain measurement using in-fiber Bragg grating sensors”, Elect. Lett. 32 (12) 1133 (1996)). In particular, the use of laser active sensors based on Bragg gratings is widespread. These include DBR (Distributed Bragg Reflector) lasers (see D. Kersey et al., “Fiber Grating Sensors”>>, J. Light. Techn. Vol. 15, No. 8 (1997)) or DFB (Distributed FeedBack) lasers (see J. Hill et al., “DFB fibre-laser sensor developments”, OFS-17 Proc. SPIE Vol. 5855 p. 904 and U.S. Pat. No. 8,844,927 entitled “Optical Fiber Distributed FeedBack Laser” (1998)). The spectral purity of these lasers enables a substantial increase in sensitivity to be achieved compared with passive Bragg grating devices.
In the case of fiber Bragg grating hydrophones, the quantity to be measured is a strain applied to the sensor. The required sensitivity is such that, whatever the type of fiber grating used (DBR, DFB, passive Bragg), the interrogation system is complex. This is because the strain on the sensor induces a phase shift on the optical wave which propagates therein. To measure this phase shift requires comparing the phase of the signal in question with a reference signal. Among the methods used, two technical solutions may chiefly be distinguished for obtaining a reference wave. The first solution consists in using a reference wave coming from a second sensor similar to the first but isolated from interference. This method is described in the article by C. Sun et al., “Serially multiplexed dual-point fiber-optic acoustic emission sensor”, J. Light. Techn. Vol. 22, No. 2 (2004). The second solution consists in splitting the signal of interest into two arms of very different optical paths and in making these two arms interfere with each other. In this case, the reference wave is a retarded copy of the signal wave. The reader may refer to the publication by S. Abad et al., “Interrogation of wavelength multiplexed fiber Bragg gratings using spectral filtering and amplitude-to-phase optical conversion”, J. of Light. Techn. Vol. 21, No. 1 (2003) for all information about this second method.
The use of active sensors emitting two optical waves of different frequencies is one conceivable solution for dispensing with interferometer benches or an additional sensor. DFB-FLs (Distributed FeedBack Fiber Lasers) oscillating on two polarization states or two propagation modes, whether transverse or longitudinal, have already formed the subject of patents and publications. Mention may be made of the following patents: U.S. Pat. No. 5,844,927 from Optoplan (Norway) 1998 “Optical fiber DFB laser”; U.S. Pat. No. 6,885,784 from Vetco Gray Controls Ltd (UK) 2005 “Anisotropic DFB fiber laser sensor”; and U.S. Pat. No. 6,630,658 from ABB Research Ltd (Switzerland) 2003 “Fiber laser pressure sensor” and the publication by Kumar et al., “Studies on a few-mode fiber-optic strain sensor based on LP01-LP02 mode interference”, J. Light. Techn. Vol. 19, No. 3 (2001).
Starting from these principles, various laser DFB fiber hydrophone architectures have been proposed. Details of these will be found in the following publications: P. E. Bagnoli et al., “Development of an erbium-doped fibre laser as a deep-sea hydrophone”, J. of Optics A: Pure Appl. Opt. 8 (2006); D. J. Hill et al., “A fiber laser hydrophone array”, SPIE Conference on Fiber Optic Sensor Technology and Applications Vol. 3860, 55 (1999); or S. Foster et al., “Ultra thin fiber laser hydrophone research through government-industry collaboration” OFS 2005-2006.